Sanding it all off with 400 will take 'forever'. Plus, it would be very hard to keep the bodywork straight doing that too. How many coats of paint are on it now? 3-4? If so, sanding it would take off a layer, then left with 2-3 - no need to take it all off. Block out the runs, sand it (wet or dry) and spray it with BC/CC. If you sand through, put a little primer on, sand it and then paint it. If the only thing wrong with the actual paint is the runs, why take it all off?